In prior-art diverting pulleys the axle of the diverting pulley is locked into position with a locking plate that extends into a groove made in the axle. The groove is milled into the axle from the transverse direction, in which case the groove comprises a detent surface as viewed in the transverse direction of the axle, against which the locking plate is placed to lock the axle in position so that it does not rotate and does not move in the axial direction. A problem in these prior-art solutions is that when the locking plate breaks and allows the axle to rotate, movability of the axle is also enabled in the axial direction. Another problem in these prior-art solutions is that since the groove is milled in the solutions at a distance from the end of the axle, the detent surface that is against the locking plate has remained hidden behind the end of the axle. That being the case, it has not been possible to visually inspect the contact point of the locking plate and the detent surface of the axle. Inspecting this point is necessary so that, among other things, the condition of the locking plate can be determined and indirectly also the condition of the bearings. If servicing intervals are long, the condition of bearings can deteriorate over time so that the diverting pulley rotating on the axle starts essentially to try to rotate the axle. When the axle tries to rotate, the pressure exerted on the locking plate by the detent surface and also the wear resulting from to-and-fro loading increase. Verifying the condition of the locking plate has been laborious in prior art, because the diverting pulley structure has had to be opened for this purpose.